The invention concerns a method and a device for directly producing a lost mold for metal castings and for fabricating casting prototypes and odd parts in the range of small- to medium-sized articles and production runs by machining a sand mold made with the use of curable binders.
Lost molds for casting prototypes and odd parts are made according to DE 43 41 325 A 1 as foam patterns by means of a numerically controlled milling machine. The numerically controlled milling machine includes a cutter that can be moved in the three spatial directions by means of a portal arrangement. To ensure fast and accurate machining of the workpiece, the polystyrene-foam particles milled from a foam block are exhausted directly from the area of the cutter. To suction off remaining stock, the cutter is implemented as a hollow body with exhaust openings and is connected by lines to an exhaust system. Since the tool costs involved in manufacturing the cutter are comparatively high, foundry-sand casting molds cannot be made by this method. The high wear occasioned by machining a lost mold would adversely affect accuracy of shape and would entail substantial tool costs. In addition, foam patterns are not suitable for small castings. Thin-walled and complex structures are difficult to make. The extreme brittleness of the pattern is a disadvantage, and its elasticity can lead to large dimensional deviations. The foundry must be specially equipped to ensure environmental safety.
To be able to produce a sand mold without a master pattern, it was proposed in DE 26 05 687 C 3 to ram the molding sand into a molding box and to hollow out the mold cavity with cutting and milling tools operatively connected to a profiling or copying milling machine. To reduce tool wear, it was further proposed that the sand block be hollowed out when the compressive strength was 2-10 kg/cm2, preferably 2-5 kg/cm2. This means that the machining must be done before the binder is completely set, but when acceptable strength has nevertheless been attained. This approach is impossible to control technologically and is associated with numerous sources of error. For this reason, the machining of uncured sand molds has failed to win acceptance in practice. Moreover, the tool used to hollow out the sand block, which is detailed in the description of the invention, comprises a knife rotating about a vertical axis, by means of which only ordinary cylindrical shapes can be produced.
German Unexamined Patent Application DE 196 49 428 A1 offered a proposal for producing molds for large and complex castings without the use of a prototype pattern set, which consisted in machining a sand block with compressive strength in the range of 20 to 80 kg/cm2, particularly 40 to 60 kg/cm2. Since a casting mold made of foundry sand containing a conventional proportion of binder and cured under normal conditions ordinarily has a compressive strength of at least 60 kg/cm2, the range of compressive strength values cited as preferable relates to an incompletely cured sand mold or to a mold with a comparatively low binder content, in order to increase tool life. Either approach is beset by major disadvantages in exact mold production, owing to the low strength of the mold.
The method is characterized by the fact that a block of compacted powdered granulate material is prepared with an automatic processing machine. A tool-changer and an exhaust system for the removed stock in the vicinity of the machining operation are cited. The powdered granulate is specified as a special sand material cemented in the form of a right parallelepipedal block by means of binders such as water glass cured with CO2 gas or hardeners such as furan resin cured with oxygen, and is present in compacted form. The processing machine has a type of machine-tool control. CAM software is used to generate a program on the basis of which the machining is performed. The program for the mold to be produced can be created on the basis of drawings or from CAD data.
The disadvantages of directly machining a block of sand material by means of the proposed automatic processing machine lie in the high tool wear manifested in the patent, a phenomenon that can be reduced only by lowering the strength of the sand block. The machining must therefore be done within the curing time, which is likewise very problematic for technical reasons. A casting mold that utilizes a rapidly curing mixture of mold materials with a very short curing time is therefore impracticable. The proposed solution is limited to the preferred use of CO2 or furan-resin mold materials, and thus not all of the candidate foundry mold materials can be used.
The invention, which is intended to avoid or palliate the aforesaid disadvantages of the prior art, is directed to a method and a device for directly producing a lost mold by machining a sand mold made nonrestrictively with the use of curable binders. In particular, the invention is intended to reduce tool wear and to minimize the attendant tool costs, as well as to increase dimensional accuracy and exactness in the reproduction of the mold.
The object is accomplished by means of the machining method of direct milling of the mold material which is characterized in that a mold blank is produced by means of a pattern set with the use of a variable or fixed molding box, said mold blank having the contours of the casting mold plus a machining allowance for the milling of the mold material. After the molding box has been filled with the curable foundry-mold materials and curing and removal from the mold have taken place, the resulting mold blank is transformed into the casting mold by high-speed machining, milling or 3-D machining of freeform surfaces.
The proposed machining method of direct milling of the mold material yields a novel set of properties for casting molds made with known high-speed-machining technologies, including quantitative and qualitative properties which, according to a further feature of the invention, are attributable to the fact that the mold blank is machined in the cured state, in which its compressive strength is at least 60 to 80 kg/cm2.
Characteristic of the milling of the mold material, therefore, is the production of mold contours in cured foundry mold material. The accumulation of removed stock in the machining area is detrimental to automation of the milling operation. The consequences are inaccuracies in the surface contour of the casting mold, increased tool wear, or damage to the machine. The production of mold blanks keeps stock removal to a minimum. Removed stock tends not to accumulate. The production of mold blanks has further advantages characterized by economization of mold material and energy, minimization of waste and dust, minimization of wear on tools and machines and reduction of machining times. In addition, particular economic efficiency is achieved in the case of large-volume molds and castings.
It is further provided within the scope of the invention that the term xe2x80x9cdirect milling of mold materialxe2x80x9d encompasses not only the milling of foundry mold materials per se, but also a technological series of processes and devices for producing lost molds. This includes the production of mold blanks by means of a variable pattern set from an arrangement of parallelepipedal segments, cubic segments, threaded bolts or clampable round rods, as well as the evacuation of removed stock. A prerequisite for the industrial application of direct milling of mold material is the availability of specialized equipment for making the mold blanks and for machining them in the processing machine, which will be explained in more detail hereinbelow by means of several exemplary embodiments. Further advantages and embodiments of the invention will emerge from the dependent claims and the exemplary embodiments.